a) Field of the Invention
This invention relates to cellulose based products and more particularly relates to viscose type products which can be formed into sausage casings.
b) History of the Prior Art
Viscose has been used for an extended period of time in the manufacture of both reinforced and unreinforced regenerated cellulose food, e.g., sausage casings. When reinforced material was made a fiber web was impregnated with the viscose to form the casing product. Presizing or precoating of the web was desirable to increase web strength and adhesion to the impregnating viscose. Xanthate viscose itself has been tried as such a precoating material.
In any case, the viscose, dissolved modified cellulose, which has been commercially used in the prior art has almost universally been formed by treating cellulose with caustic soda and carbon disulfide to form cellulose xanthate which is then dissolved in weak caustic solution to form the viscose. The products formed from cellulose regenerated from this viscose have found great commercial success. Unfortunately, the carbon disulfide used in the process and by-product carbon disulfide and hydrogen sulfide from the process are flammable and extremely toxic and these products must be carefully managed. Such management is made more complex when the material is a tubular material such as a sausage casing which can conduct gaseous products.
In addition, in the traditional viscose process for the manufacture of sausage casings, regeneration of the cellulose is necessary. Furthermore, the resulting cellulose product when in the form of film does not lend itself to internal plasticization and requires some kind of plasticizer for handling. In the absence of some kind of plasticizer the product is brittle.
A viable alternate to the traditional viscose process for the manufacture of sausage casings would therefore be desirable.
As early as 1930 (U.S. Pat. No. 1,771,461) it was proposed that ammonia derivatives of carbon dioxide such as urea, could be reacted with cellulose to form soluble products which could subsequently be used for the manufacture of fibers and films. This process was further discussed in U.S. Pat. Nos. 2,129,708 (1938) and 2,134,825 (1938) assigned on their faces to E. I. DuPont. The viscose type products resulting from this process are esters which will be referred to herein as cellulose aminomethanates, although they may also be known as cellulose amino formates, cellulose carbamates or cellulose aminomethanoates in other references.
While the resulting final products, e.g. fibers and films, at least when made on a small scale, had fair properties, the properties, especially purity strength and solubility at comparable chain lengths, were not nearly as good as similar products made from conventional viscose, i.e. the xanthate process. Recently, in part due to increased awareness of our environment, interest has again been shown in the alternate viscose technology disclosed in the above early references. It has, for example, been disclosed in U.S. Pat. No. 4,404,369 issued in 1983, that an alkali-soluble cellulose derivative could be produced by treating cellulose with liquid ammonia having urea dissolved therein. The object was to develop a product having urea distributed through the product prior to reaction by heating. The process described nevertheless has problems in that liquid ammonia also must be contained and in addition the product still does not have properties as good as desired for many commercial applications
Various proposals have been made for increasing solubility of the cellulose aminomethanate product, e.g. U.S. Pat. No. 4,526,620 wherein excess urea is used to increase solubility but simultaneously creates urea contamination and U.S. Pat. No. 4,530,999 where the chain length is reduced by radiation which unfortunately also decreases end product strength.
It was proposed in European Patent Publication 178,292 published Apr. 16, 1986, that an improved product could be obtained when an alkali-urea impregnated cellulose was washed with urea solution to remove hydroxide prior to heating to form the ester. While this provided some improvement in the properties of the resulting ester, uniformity and thus strength especially when large quantities of products were made, are still not as good as desired to permit substitution for most xanthate type viscose in most commercial applications.